In recent years, techniques related to augmented reality (AR) are actively being developed. AR is a collective term of techniques for expanding, by computers, real environments perceived by people.
As an example of AR, the following technique is known: the technique for using computer graphics or the like to superimpose additional information on an image (hereinafter referred to as acquired image) acquired by a camera connected to a mobile information terminal device such as a portable personal computer, a tablet, or a smartphone, displaying the acquired image and the additional information superimposed on the acquired image on a display screen of the mobile information terminal device, and achieving support for a user's task (refer to, for example, Hirotake Ishii, Proposal and Evaluation of Decommissioning Support Method of Nuclear Power Plants using Augmented Reality, Journal of the Virtual Reality Society of Japan, Vol. 13, No. 2, pp. 289-300, June, 2008).
In order to superimpose and display the additional information at an appropriate position, the position and orientation of the camera is estimated. The accuracy of the estimation is very important. As a method of estimating the position and orientation of the camera, techniques for estimating the position and orientation of the camera based on the acquired image are known.
As one of the techniques for estimating the position and orientation of the camera based on the acquired image, a method using characteristic points included in the acquired image is known (refer to, for example, Japanese Laid-open Patent Publication No. 2005-326274). Each of the characteristic points is normally a point detected as a standard by uniquely defining the position of the target point based on a variation in a gray level in a region located near the target point, while the variation in the gray level in the region located near the target point is large.
The method of estimating the position and orientation of the camera using the characteristic points is to estimate the position and orientation of the camera by using calculated three-dimensional coordinates of characteristic points (hereinafter referred to as map points) to associate the characteristic points within the acquired image to be processed with the map points in a matching process.